In experiments in unanesthetized horses in which arterial pressure (PA) has been changed by the intravenous administration of vasoactive drugs, it was found that: 1) Cardiac output (CO) increased in a linear fashion with a decrease in PA and CO decreased in a linear fashion with an increase in PA; the slope of the CO vs PA line in the hypertensive region was greater than in the hypotensive region. 2) Heart rate (HR) behaved similar to CO except that the HR vs PA slope was greater in hypotension than in hypertension. 3) Stroke volume (SV) decreased as PA was decreased from resting and also decreased as PA was increased from resting. The power transfer (W) between the left ventricle and arteries was maximal from resting. The power transfer (W) between the left ventricle and arteries was maximal valued at resting levels of total peripheral resistance. Apparently, the resting vasoactive state is optimal with respect to energy transfer between the left ventricle and systemic arteries and mechanical efficiency of the heart.